Potency ratio between opioid agonists and partial agonists.

POTENCY RATIO BETWEEN OPIOID AGONISTS AND PARTIAL AGONISTS Sir,—The paper by Klepper and colleagues (1986) again seems to indicate the belief of this group of workers in a fixed potency ratio between opioid agonists and partial agonists, although such a concept is theoretically unlikely (Kay, 1985). Support for a variable potency ratio is apparent in their paper, however, in that nalbuphine in so-called "equipotent" doses smaller than those in the range (8-10 mg) where equipotency has been demonstrated (Beaver and Feise, 1978) produced a greater effect than morphine. This observation correlates with the theoretical prediction based on nalbuphine having a flatter dose-response relation than morphine, as docs the observation that doses of nalbuphine in excess of 10 mg produced less effect than "equipotent" doses of morphine. The paper confirms that nalbuphine has a flatter dose—response relation than morphine, and that a ceiling effect, or even reversal of effect, occurs. With non-parallel dose-response relations, any potency ratio between nalbuphine and morphine can only apply at a single dose.